This invention relates to a method of making intaglio halftone gravure printing plates having improved tone reproducibility.
In offset printing and relief printing, the tone of an original is reproduced in accordance with differences in area of halftone dots.
On the other hand, in gravure printing, the areas of halftone dots are, in general, the same, and the shapes thereof are square. However, the dots or cells in the printing surface are different in depth, and the depth of each dot or cell determines the amount of ink to be carried therein, that is, the amount of ink carried in each dot is proportional to the depth thereof, whereby the tone of an original is reproduced in accordance with the variations in amount of ink carried in the dots. This gravure printing is well known in the art as conventional gravure printing. In addition, well known in the art is another kind of gravure printing in which the tone of an original is reproduced by both the depth and the area of halftone dots. This has been provided in order to prevent variation of tone and to improve durability of the printing plate, and is called intaglio halftone gravure printing.
An ordinary method of making a printing plate for the gravure printing is the so-called "Dultgen process" in which a continuous-tone positive and a halftone positive are employed. In this method, a sensitized carbon tissue is exposed through the halftone positive, and the continuous-tone positive is printed on the non-exposure areas of the carbon tissue thus exposed. The carbon tissue thus treated is applied onto a printing plate or cylinder, which is subjected to etching after developing of the carbon tissue.
This process is advantageous in stability in making the printing surface, but is disadvantageous in that the size of the continuous-tone positive must be exactly the same as that of the halftone positive, which means that the method is relatively intricate and is difficult from the technical viewpoint. Furthermore, in this known method, light tone areas of the original, especially the light tone areas having halftone dot percentages of the order of 5 to 8% are not reproduced, and therefore the printed matter appears rough, that is, it lacks smooth tone in its entirety.